Needs of the marketplace for printers are increased every year. Regarding color image formation, a 4-color image forming apparatus using 4 color ink or toner is popular, however, image forming apparatuses using increased number of colors of ink or toner have been proposed and realized. For example, an ink-jet printer which performs image formation using 6 inks including light cyan and light magenta inks as well as conventional cyan, magenta, yellow and black inks for the purpose of mitigation of graininess has been realized. Further, an electrophotographic printer using 6 colors as in the case of the ink-jet printer has been proposed (Japanese Patent Application Laid-Open Nos. 2001-290319 and 2001-318499). Additionally, an electrophotographic printer using green and orange inks for enlargement of color gamut has been proposed and realized. Further, a printer which performs image formation using ink toner corresponding to special colors such as red, blue, green or gold, silver and fluorescent colors has been proposed and realized.
In this manner, various types of image forming apparatuses using increased number of ink/toner colors are known, however, in these apparatuses, the use of increased number of ink/toner colors is intended for improvement in color reproducibility of an image output device itself.
On the other hand, edge smoothing is known as disclosed in Japanese Patent Application Laid-Open No. 08-317210. Edge smoothing is an image processing technique mainly used in electrophotographic printers. In this technique, an edge included in an image signal is detected, and the edge portion or edge-adjacent portion is replaced with halftone dots, thereby jaggies on character and thin line images are reduced, and smoothed edges are reproduced on a paper medium.
The edge smoothing technique performs independent processing for each color channel in the conventional 4-color (CMYK) image forming apparatus. To apply the edge smoothing technique to the above-described image forming apparatus using the dark and light color recording materials, a circuit to realize smoothing processing is provided for the respective color recording materials. Accordingly, smoothing processing is performed on even light color signals. However, the application of smoothing processing to a light color signal, where edge portions are not conspicuous, does not greatly improve the image quality, but increases the circuit scale, thus increases the costs of the apparatus.
Particularly, in the electrophotographic image forming apparatuses, there is a close correspondence between the size of a dot to be formed and the stability in dot formation. That is, when a large dot is formed so as to obtain a higher density, the stability of dot formation is increased, while when a small dot is formed so as to obtain a low density, the stability of dot formation is lowered (becomes unstable). As dots added to an edge portion in the edge smoothing processing are intermediate density dots, the stability of the dots are lower than full dots.